One of known methods for determining the dielectric permittivity of a material consists of irradiating a sample with an electromagnetic wave using a double-arm emitter, changing difference between the signal's phases in the arms of the emitter and measuring amplitude of the wave transmitted at an angle and determining the dielectric permittivity. By changing the difference between the signal's phases in the arms of the emitter, the dependence of the amplitude of the transmitted wave on the length of the arm is eliminated. The dielectric permittivity is determined by the following formula:
      ɛ    =                            λ          0          2                                      Sin            2                    ⁢          Q                    ·                        (                                    1              Δ                        -                          1                              λ                b                                              )                2              ,where λo—free-space wavelength; λb—wavelength in the double-armed emitter; Δ—period of amplitude “zero” for the transmitted wave, and angle θ is chosen according to the relationship
            (                                    d            k                                λ            b                          -        1            )        <                            d          k                          λ          0                    ⁢              ɛ            ⁢      Sin      ⁢                          ⁢      θ        <          (                                    d            k                                λ            b                          +        1            )        ,where dk—maximum size of the emitter's arm, see USSR Patent No. SU 1800333 A1.
The disadvantage of this method is that it requires a contact between the emitter and the object, the dielectric permittivity of which is to be determined. Moreover, this sample needs to have a flat surface to ensure a contact with the emitter. These requirements do not allow using this method for remote determination of the dielectric permittivity of objects.
Another known method to determine the dielectric permittivity of a dielectric object employs irradiating the dielectric object with coherent microwave radiation at N frequencies. The irradiation is carried out against the background of reflectors, whereby boundaries between the object's layers, or a boundary between the dielectric object and air, or a physical body, on which the tested dielectric object is placed, serve as the reflectors. The signal reflected from the dielectric object and the reflector is registered. The detected signals are converted in the time domain. Peak temporal components in the temporal spectrum are determined and the times of the determined peak temporal components are measured. These data are used to determine the dielectric permittivity and the thickness of the layers. Probing and receiving is carried out in a sector of angles. Dielectric permittivity and thickness of layers are then determined from the formulae:
                    ⁢                            ɛ          i                =                                                            ɛ                1                                      ⁢            Sin            ⁢                                                  ⁢                          θ                              na                ⁢                                  ∂                  1                                                            (                i                )                                      *            c            *                          (                                                t                  i                                -                                                      2                    c                                    ⁢                                                            ∑                                              p                        =                        1                                                                    i                        -                        1                                                              ⁢                                                                                                                        ɛ                            p                                                                          ⁢                        Δ                        ⁢                                                                                                  ⁢                                                  1                          p                                                                                                                      1                          -                                                                                                                    ɛ                                1                                                            /                                                              ɛ                                p                                                                                      *                                                          Sin                              2                                                        ⁢                                                          θ                                                              na                                ⁢                                                                  ∂                                  1                                                                                                                            (                                i                                )                                                                                                                                                                                                                    )                                            2            *                          (                                                d                  2                                -                                                      ∑                                          p                      =                      1                                                              i                      -                      1                                                        ⁢                                      Δ                    ⁢                                                                                  ⁢                                          1                      p                                        *                                                                                            ɛ                          1                                                ⁢                        Sin                        ⁢                                                                                                  ⁢                                                  θ                                                      na                            ⁢                                                          ∂                              1                                                                                                            (                            i                            )                                                                                                                                                                            ɛ                            p                                                    -                                                                                    ɛ                              1                                                        ⁢                                                          Sin                              2                                                        ⁢                                                          θ                                                              na                                ⁢                                                                  ∂                                  1                                                                                                                            (                                i                                )                                                                                                                                                                                                                    )                                          ;                          Δ        ⁢                                  ⁢                  1          i                    =                        (                                    t              i                        -                                          2                c                            ⁢                                                ∑                                      p                    =                    1                                                        i                    -                    1                                                  ⁢                                                                                                    ɛ                                                  p                          1                                                                                      *                    Δ                    ⁢                                                                                  ⁢                                          1                      p                                                                                                  1                      -                                                                                                    ɛ                            1                                                    /                                                      ɛ                            p                                                                          *                                                  Sin                          2                                                ⁢                                                  θ                                                      na                            ⁢                                                          ∂                              1                                                                                                            (                            i                            )                                                                                                                                                                            )                *                              c            *                                          1                -                                                                            ɛ                      1                                                              ɛ                      i                                                        ⁢                                      Sin                    2                                    ⁢                                      θ                                          pa                      ⁢                                              ∂                        1                                                                                    (                      i                      )                                                                                                                2            ⁢                                          ɛ                i                                                          ,  where i—the number of the layer; ∈i and ∈p—dielectric permittivity of layers i and p; ∈1—the dielectric permittivity of the medium in which probing and receiving of signals is performed; Δli—thickness of i-layer;
      Δ    ⁢                  ⁢    1    =                    h        1            +              h        2              2  where h1 and h2 are heights between the border of the first and second layers to the points from which probing is performed and the signal receiving points respectively; θna∂1(i) is angle of received signal reflected from the border between layers i and i+1, c—speed of light; ti—frequency of peak i-constituent of the time spectrum which corresponds to the reflection of the signal from the border between layers i and i+1, and d—projection on the probe surface of the distance between the point of probing and the signal receiving point, see Russian Patent No. RU 2039352 C1.
The disadvantage of this method, which is taken as a prototype for the present invention, is the requirement of parallel arrangement of the layers of the dielectric object. If the object consists of a single layer, its sides should be parallel. Therefore this method can be used solely for custom-made objects with required features. Moreover, this method also requires that the angles of incidence and reflection of the microwave radiation towards the dielectric object are well-defined.
The aforementioned renders it impossible to use this method in practice for determining dielectric permittivity of a moving and hidden object with non-parallel layers or sides, particularly for covert detection of the presence of dielectric explosive compounds hidden on a human body. It is known that dielectric permittivity of the vast majority of such compounds lies between 2.9-3.1.